The present invention relates to monitoring systems and more particularly to a current-to-current electrical signal isolator for use in such systems.
In a process plant, such as an oil refinery or an atomic reactor facility, a large number of process variables are simultaneously monitored by the operating personnel. For example, such a process plant may have hundreds of sensing transducers, such as temperature responsive thermocouples, which constantly monitor various portions of the processes. Information from the sensing transducers, in the form of electrical signals, is communicated to one or more locations where they may be connected to various monitoring instruments. For example, the signals from the various sensors may be communicated to a central control room and utilized in control room instruments such as meters, chart recorders, controllers, computers or annunciator warning devices.
At the present time, in one type of monitoring system utilized in a process plant, each transducer is connected by wires to a transmitter. The transmitter is connected to a power source and is also connected, by output wires, to the control location. It frequently occurs that one or more electrical groundloop problems occur when the transmitters are connected to the control room instruments. One possible problem arises when more than one control room instrument is connected to a single transmitter. For example, a chart recorder and an analog-to-digital converter may both be desired to be connected to a single transmitter so that both receive information from the same sensor. If one of the control room instruments, such as the analog-to-digital converter, is connected to ground, then errors in measurement may result. The control room instruments may be grounded, because they operate from 24 volts direct current, or as a safety measure. Another problem is that the sensing transducer may be grounded; for example, some thermocouples are purposely grounded to improve their mechanical stability and others become grounded after they age or become damaged. Still another problem is that there is a lack of standardization as to the ground connections of the control room instruments; some may be positive ground, others may be negative ground, and still others may have an intermediate ground potential. The manufacturer of the control room instrument may not state the exact ground required by the instrument, or the instructions may be lost, so that the grounding problem may not be recognized until the instrument is fully connected. Any of these differences in ground potential between the sensors or transmitters and the control room instruments may cause circulating currents which disturb and distort the signals received by the control room instruments.
It is known that a solution to the grounding problems which may occur in a process monitoring system may be obtained by utilizing a current-to-current isolator. If the current-to-current isolators are incorporated into the transmitters then they may derive their operating power from the same source as the transmitter. However, to use a current-to-current isolator with each transmitter is economically unjustified in many situations. A process plant may have hundreds of transmitters and there may be ground potential problems in only a few of their loops; for example, problems may occur in only 10% of the transmitter loops. The other transmitter loops may not present any grounding problems, although their sensor or control room instrument is grounded, because the control room instrument is physically close to the sensing transducer and consequently there may not be appreciable ground potential differences. In that situation the current-to-current isolators in 90% of the transmitters would serve no useful function and yet they would add to the cost and size of all of the transmitters.
It is presently known that the preferred solution to grounding problems in a process plant monitoring system is the use of separate current-to-current isolators. Such current-to-current isolators may be utilized in only those transmitter loops which present grounding problems or otherwise require electrical isolation. Presently. such isolators are powered by the plant power lines, usually, in the United States, a 110-volt a.c. line, and include a power regulator and separate input and output amplifiers. However, due to the relatively high cost of skilled electrical labor, it may be relatively expensive to connect the isolator to the power line, for example, because the isolator may be far from a power line or otherwise inconveniently located. In addition, such current-to-current isolators are expensive and are relatively bulky, for example, over 100 cubic inches, which may present problems in mounting them in a convenient location. Generally such isolators are "retrofitted", i.e., utilized in existing installations.